Vulcanization of diene polymers generally involves using powdered sulfur, insoluble sulfur, or the like, or hybrid crosslinking agents which allow for reduction in sulfur content, as well as vulcanization accelerators, zinc oxide, fatty acids, and other agents. Using sulfur-containing powder or liquid compounds in place of powdered sulfur is also proposed.
In other known techniques, the amount of sulfur is reduced, and resorcin or phenol resins and formalin are used to reinforce crosslinking. However, these resins are too polar to penetrate into the polymer matrix, and it is difficult to obtain satisfactory elongation at break, abrasion resistance, and other properties.
Thus, since the affinity for diene polymers such as styrene butadiene rubber, polybutadiene rubber, and natural rubbers is still low in the conventional techniques, it is difficult to uniformly distribute elemental sulfur in rubber. For this reason, a sufficient amount of sulfur crosslinks cannot be formed more uniformly in the polymer matrix, and therefore good tensile properties cannot be obtained. In addition, the tire properties such as durability of grip during running and running performance are not satisfactory, and further improvement is needed.